BUG#21051: RESET QUERY CACHE very slow when query_cache_type=0

There were two problems: RESET QUERY CACHE took a long time to complete
and other threads were blocked during this time.

The patch does three things:
  1 fixes a bug with improper use of test-lock-test_again technique.
      AKA Double-Checked Locking is applicable here only in few places.
  2 Somewhat improves performance of RESET QUERY CACHE.
      Do my_hash_reset() instead of deleting elements one by one.  Note
      however that the slowdown also happens when inserting into sorted
      list of free blocks, should be rewritten using balanced tree.
  3 Makes RESET QUERY CACHE non-blocking.
      The patch adjusts the locking protocol of the query cache in the
      following way: it introduces a flag flush_in_progress, which is
      set when Query_cache::flush_cache() is in progress.  This call
      sets the flag on enter, and then releases the lock.  Every other
      call is able to acquire the lock, but does nothing if
      flush_in_progress is set (as if the query cache is disabled).
      The only exception is the concurrent calls to
      Query_cache::flush_cache(), that are blocked until the flush is
      over.  When leaving Query_cache::flush_cache(), the lock is
      acquired and the flag is reset, and one thread waiting on
      Query_cache::flush_cache() (if any) is notified that it may
      proceed.


include/mysql_com.h:
  Add comment for NET::query_cache_query.
sql/net_serv.cc:
  Use query_cache_init_query() for initialization of
  NET::query_cache_query if query cache is used.
  Do not access net->query_cache_query without a lock.
sql/sql_cache.cc:
  Fix bug with accessing query_cache_size, Query_cache_query::wri and
  thd->net.query_cache_query before acquiring the lock---leave
  double-check locking only in safe places.
  Wherever we check that cache is usable (query_cache_size > 0) we now
  also check that flush_in_progress is false, i.e. we are not in the
  middle of cache flush.
  Add Query_cache::not_in_flush_or_wait() method and use it in
  Query_cache::flush_cache(), so that threads doing cache flush will
  wait it to finish, while other threads will bypass the cache as if
  it is disabled.
  Extract Query_cache::free_query_internal() from Query_cache::free_query(),
  which does not removes elements from the hash, and use it together with
  my_hash_reset() in Query_cache::flush_cache().
sql/sql_cache.h:
  Add declarations for new members and methods.
  Make is_cacheable() a static method.
  Add query_cache_init_query() function.
sql/sql_class.cc:
  Use query_cache_init_query() for initialization of
  NET::query_cache_query.

include/mysql_com.h

@@ -210,7 +210,13 @@ typedef struct st_net {
   char last_error[MYSQL_ERRMSG_SIZE], sqlstate[SQLSTATE_LENGTH+1];
   unsigned int last_errno;
   unsigned char error;
+
+  /*
+    'query_cache_query' should be accessed only via query cache
+    functions and methods to maintain proper locking.
+  */
   gptr query_cache_query;
+
   my_bool report_error; /* We should report error (we have unreported error) */
   my_bool return_errno;
 } NET;

sql/net_serv.cc

@@ -96,8 +96,11 @@ extern uint test_flags;
 extern ulong bytes_sent, bytes_received, net_big_packet_count;
 extern pthread_mutex_t LOCK_bytes_sent , LOCK_bytes_received;
 #ifndef MYSQL_INSTANCE_MANAGER
-extern void query_cache_insert(NET *net, const char *packet, ulong length);
+#ifdef HAVE_QUERY_CACHE
 #define USE_QUERY_CACHE
+extern void query_cache_init_query(NET *net);
+extern void query_cache_insert(NET *net, const char *packet, ulong length);
+#endif // HAVE_QUERY_CACHE
 #define update_statistics(A) A
 #endif /* MYSQL_INSTANCE_MANGER */
 #endif /* defined(MYSQL_SERVER) && !defined(MYSQL_INSTANCE_MANAGER) */
@@ -133,7 +136,11 @@ my_bool my_net_init(NET *net, Vio* vio)
   net->compress=0; net->reading_or_writing=0;
   net->where_b = net->remain_in_buf=0;
   net->last_errno=0;
-  net->query_cache_query=0;
+#ifdef USE_QUERY_CACHE
+  query_cache_init_query(net);
+#else
+  net->query_cache_query= 0;
+#endif
   net->report_error= 0;
 
   if (vio != 0)					/* If real connection */
@@ -552,9 +559,7 @@ net_real_write(NET *net,const char *packet,ulong len)
   my_bool net_blocking = vio_is_blocking(net->vio);
   DBUG_ENTER("net_real_write");
 
-#if defined(MYSQL_SERVER) && defined(HAVE_QUERY_CACHE) \
-                          && !defined(MYSQL_INSTANCE_MANAGER)
-  if (net->query_cache_query != 0)
+#if defined(MYSQL_SERVER) && defined(USE_QUERY_CACHE)
   query_cache_insert(net, packet, len);
 #endif
 

sql/sql_cache.cc

@@ -564,22 +564,63 @@ byte *query_cache_query_get_key(const byte *record, uint *length,
   Functions to store things into the query cache
 *****************************************************************************/
 
+/*
+  Note on double-check locking (DCL) usage.
+
+  Below, in query_cache_insert(), query_cache_abort() and
+  query_cache_end_of_result() we use what is called double-check
+  locking (DCL) for NET::query_cache_query.  I.e. we test it first
+  without a lock, and, if positive, test again under the lock.
+
+  This means that if we see 'NET::query_cache_query == 0' without a
+  lock we will skip the operation.  But this is safe here: when we
+  started to cache a query, we called Query_cache::store_query(), and
+  NET::query_cache_query was set to non-zero in this thread (and the
+  thread always sees results of its memory operations, mutex or not).
+  If later we see 'NET::query_cache_query == 0' without locking a
+  mutex, that may only mean that some other thread have reset it by
+  invalidating the query.  Skipping the operation in this case is the
+  right thing to do, as NET::query_cache_query won't get non-zero for
+  this query again.
+
+  See also comments in Query_cache::store_query() and
+  Query_cache::send_result_to_client().
+
+  NOTE, however, that double-check locking is not applicable in
+  'invalidate' functions, as we may erroneously skip invalidation,
+  because the thread doing invalidation may never see non-zero
+  NET::query_cache_query.
+*/
+
+
+void query_cache_init_query(NET *net)
+{
+  /*
+    It is safe to initialize 'NET::query_cache_query' without a lock
+    here, because before it will be accessed from different threads it
+    will be set in this thread under a lock, and access from the same
+    thread is always safe.
+  */
+  net->query_cache_query= 0;
+}
+
+
 /*
   Insert the packet into the query cache.
-  This should only be called if net->query_cache_query != 0
 */
 
 void query_cache_insert(NET *net, const char *packet, ulong length)
 {
   DBUG_ENTER("query_cache_insert");
 
+  /* See the comment on double-check locking usage above. */
+  if (net->query_cache_query == 0)
+    DBUG_VOID_RETURN;
+
   STRUCT_LOCK(&query_cache.structure_guard_mutex);
-  /*
-    It is very unlikely that following condition is TRUE (it is possible
-    only if other thread is resizing cache), so we check it only after guard
-    mutex lock
-  */
-  if (unlikely(query_cache.query_cache_size == 0))
+
+  if (unlikely(query_cache.query_cache_size == 0 ||
+               query_cache.flush_in_progress))
   {
     STRUCT_UNLOCK(&query_cache.structure_guard_mutex);
     DBUG_VOID_RETURN;
@@ -616,10 +657,10 @@ void query_cache_insert(NET *net, const char *packet, ulong length)
     header->result(result);
     header->last_pkt_nr= net->pkt_nr;
     BLOCK_UNLOCK_WR(query_block);
+    DBUG_EXECUTE("check_querycache",query_cache.check_integrity(0););
   }
   else
     STRUCT_UNLOCK(&query_cache.structure_guard_mutex);
-  DBUG_EXECUTE("check_querycache",query_cache.check_integrity(0););
   DBUG_VOID_RETURN;
 }
 
@@ -628,21 +669,20 @@ void query_cache_abort(NET *net)
 {
   DBUG_ENTER("query_cache_abort");
 
-  if (net->query_cache_query != 0)	// Quick check on unlocked structure
-  {
+  /* See the comment on double-check locking usage above. */
+  if (net->query_cache_query == 0)
+    DBUG_VOID_RETURN;
+
   STRUCT_LOCK(&query_cache.structure_guard_mutex);
-    /*
-      It is very unlikely that following condition is TRUE (it is possible
-      only if other thread is resizing cache), so we check it only after guard
-      mutex lock
-    */
-    if (unlikely(query_cache.query_cache_size == 0))
+
+  if (unlikely(query_cache.query_cache_size == 0 ||
+               query_cache.flush_in_progress))
   {
     STRUCT_UNLOCK(&query_cache.structure_guard_mutex);
     DBUG_VOID_RETURN;
   }
 
-    Query_cache_block *query_block = ((Query_cache_block*)
+  Query_cache_block *query_block= ((Query_cache_block*)
                                    net->query_cache_query);
   if (query_block)			// Test if changed by other thread
   {
@@ -650,11 +690,12 @@ void query_cache_abort(NET *net)
     BLOCK_LOCK_WR(query_block);
     // The following call will remove the lock on query_block
     query_cache.free_query(query_block);
-    }
-    net->query_cache_query=0;
+    net->query_cache_query= 0;
     DBUG_EXECUTE("check_querycache",query_cache.check_integrity(1););
-    STRUCT_UNLOCK(&query_cache.structure_guard_mutex);
   }
+
+  STRUCT_UNLOCK(&query_cache.structure_guard_mutex);
+
   DBUG_VOID_RETURN;
 }
 
@@ -663,37 +704,36 @@ void query_cache_end_of_result(THD *thd)
 {
   DBUG_ENTER("query_cache_end_of_result");
 
-  if (thd->net.query_cache_query != 0)	// Quick check on unlocked structure
-  {
+  /* See the comment on double-check locking usage above. */
+  if (thd->net.query_cache_query == 0)
+    DBUG_VOID_RETURN;
+
 #ifdef EMBEDDED_LIBRARY
   query_cache_insert(&thd->net, (char*)thd, 
                      emb_count_querycache_size(thd));
 #endif
+
   STRUCT_LOCK(&query_cache.structure_guard_mutex);
-    /*
-      It is very unlikely that following condition is TRUE (it is possible
-      only if other thread is resizing cache), so we check it only after guard
-      mutex lock
-    */
-    if (unlikely(query_cache.query_cache_size == 0))
+
+  if (unlikely(query_cache.query_cache_size == 0 ||
+               query_cache.flush_in_progress))
   {
     STRUCT_UNLOCK(&query_cache.structure_guard_mutex);
     DBUG_VOID_RETURN;
   }
 
-    Query_cache_block *query_block = ((Query_cache_block*)
+  Query_cache_block *query_block= ((Query_cache_block*)
                                    thd->net.query_cache_query);
   if (query_block)
   {
     DUMP(&query_cache);
     BLOCK_LOCK_WR(query_block);
-      Query_cache_query *header = query_block->query();
-      Query_cache_block *last_result_block = header->result()->prev;
-      ulong allign_size = ALIGN_SIZE(last_result_block->used);
-      ulong len = max(query_cache.min_allocation_unit, allign_size);
+    Query_cache_query *header= query_block->query();
+    Query_cache_block *last_result_block= header->result()->prev;
+    ulong allign_size= ALIGN_SIZE(last_result_block->used);
+    ulong len= max(query_cache.min_allocation_unit, allign_size);
     if (last_result_block->length >= query_cache.min_allocation_unit + len)
       query_cache.split_block(last_result_block,len);
-      STRUCT_UNLOCK(&query_cache.structure_guard_mutex);
 
 #ifndef DBUG_OFF
     if (header->result() == 0)
@@ -701,12 +741,20 @@ void query_cache_end_of_result(THD *thd)
       DBUG_PRINT("error", ("end of data whith no result. query '%s'",
                            header->query()));
       query_cache.wreck(__LINE__, "");
+
+      STRUCT_UNLOCK(&query_cache.structure_guard_mutex);
+
       DBUG_VOID_RETURN;
     }
 #endif
     header->found_rows(current_thd->limit_found_rows);
-      header->result()->type = Query_cache_block::RESULT;
+    header->result()->type= Query_cache_block::RESULT;
     header->writer(0);
+    thd->net.query_cache_query= 0;
+    DBUG_EXECUTE("check_querycache",query_cache.check_integrity(1););
+
+    STRUCT_UNLOCK(&query_cache.structure_guard_mutex);
+
     BLOCK_UNLOCK_WR(query_block);
   }
   else
@@ -714,9 +762,7 @@ void query_cache_end_of_result(THD *thd)
     // Cache was flushed or resized and query was deleted => do nothing
     STRUCT_UNLOCK(&query_cache.structure_guard_mutex);
   }
-    thd->net.query_cache_query=0;
-    DBUG_EXECUTE("check_querycache",query_cache.check_integrity(0););
-  }
+
   DBUG_VOID_RETURN;
 }
 
@@ -762,7 +808,6 @@ ulong Query_cache::resize(ulong query_cache_size_arg)
 			query_cache_size_arg));
   DBUG_ASSERT(initialized);
   STRUCT_LOCK(&structure_guard_mutex);
-  if (query_cache_size > 0)
   free_cache();
   query_cache_size= query_cache_size_arg;
   ::query_cache_size= init_cache();
@@ -784,7 +829,15 @@ void Query_cache::store_query(THD *thd, TABLE_LIST *tables_used)
   TABLE_COUNTER_TYPE local_tables;
   ulong tot_length;
   DBUG_ENTER("Query_cache::store_query");
-  if (query_cache_size == 0 || thd->locked_tables)
+  /*
+    Testing 'query_cache_size' without a lock here is safe: the thing
+    we may loose is that the query won't be cached, but we save on
+    mutex locking in the case when query cache is disabled or the
+    query is uncachable.
+
+    See also a note on double-check locking usage above.
+  */
+  if (thd->locked_tables || query_cache_size == 0)
     DBUG_VOID_RETURN;
   uint8 tables_type= 0;
 
@@ -836,9 +889,9 @@ sql mode: 0x%lx, sort len: %lu, conncat len: %lu",
      acquiring the query cache mutex.
     */
     ha_release_temporary_latches(thd);
-    STRUCT_LOCK(&structure_guard_mutex);
 
-    if (query_cache_size == 0)
+    STRUCT_LOCK(&structure_guard_mutex);
+    if (query_cache_size == 0 || flush_in_progress)
     {
       STRUCT_UNLOCK(&structure_guard_mutex);
       DBUG_VOID_RETURN;
@@ -912,11 +965,12 @@ sql mode: 0x%lx, sort len: %lu, conncat len: %lu",
 	double_linked_list_simple_include(query_block, &queries_blocks);
 	inserts++;
 	queries_in_cache++;
-	STRUCT_UNLOCK(&structure_guard_mutex);
-
 	net->query_cache_query= (gptr) query_block;
 	header->writer(net);
 	header->tables_type(tables_type);
+
+	STRUCT_UNLOCK(&structure_guard_mutex);
+
 	// init_n_lock make query block locked
 	BLOCK_UNLOCK_WR(query_block);
       }
@@ -970,12 +1024,16 @@ Query_cache::send_result_to_client(THD *thd, char *sql, uint query_length)
   Query_cache_query_flags flags;
   DBUG_ENTER("Query_cache::send_result_to_client");
 
-  if (query_cache_size == 0 || thd->locked_tables ||
-      thd->variables.query_cache_type == 0)
-    goto err;
+  /*
+    Testing 'query_cache_size' without a lock here is safe: the thing
+    we may loose is that the query won't be served from cache, but we
+    save on mutex locking in the case when query cache is disabled.
 
-  /* Check that we haven't forgot to reset the query cache variables */
-  DBUG_ASSERT(thd->net.query_cache_query == 0);
+    See also a note on double-check locking usage above.
+  */
+  if (thd->locked_tables || thd->variables.query_cache_type == 0 ||
+      query_cache_size == 0)
+    goto err;
 
   if (!thd->lex->safe_to_cache_query)
   {
@@ -1011,11 +1069,15 @@ Query_cache::send_result_to_client(THD *thd, char *sql, uint query_length)
   }
 
   STRUCT_LOCK(&structure_guard_mutex);
-  if (query_cache_size == 0)
+  if (query_cache_size == 0 || flush_in_progress)
   {
     DBUG_PRINT("qcache", ("query cache disabled"));
     goto err_unlock;
   }
+
+  /* Check that we haven't forgot to reset the query cache variables */
+  DBUG_ASSERT(thd->net.query_cache_query == 0);
+
   Query_cache_block *query_block;
 
   tot_length= query_length + thd->db_length + 1 + QUERY_CACHE_FLAGS_SIZE;
@@ -1241,14 +1303,12 @@ void Query_cache::invalidate(THD *thd, TABLE_LIST *tables_used,
 			     my_bool using_transactions)
 {
   DBUG_ENTER("Query_cache::invalidate (table list)");
-  if (query_cache_size > 0)
-  {
   STRUCT_LOCK(&structure_guard_mutex);
-    if (query_cache_size > 0)
+  if (query_cache_size > 0 && !flush_in_progress)
   {
     DUMP(this);
 
-      using_transactions = using_transactions &&
+    using_transactions= using_transactions &&
       (thd->options & (OPTION_NOT_AUTOCOMMIT | OPTION_BEGIN));
     for (; tables_used; tables_used= tables_used->next_local)
     {
@@ -1269,17 +1329,17 @@ void Query_cache::invalidate(THD *thd, TABLE_LIST *tables_used,
     }
   }
   STRUCT_UNLOCK(&structure_guard_mutex);
-  }
+
   DBUG_VOID_RETURN;
 }
 
 void Query_cache::invalidate(CHANGED_TABLE_LIST *tables_used)
 {
   DBUG_ENTER("Query_cache::invalidate (changed table list)");
-  if (query_cache_size > 0 && tables_used)
+  if (tables_used)
   {
     STRUCT_LOCK(&structure_guard_mutex);
-    if (query_cache_size > 0)
+    if (query_cache_size > 0 && !flush_in_progress)
     {
       DUMP(this);
       for (; tables_used; tables_used= tables_used->next)
@@ -1309,10 +1369,10 @@ void Query_cache::invalidate(CHANGED_TABLE_LIST *tables_used)
 void Query_cache::invalidate_locked_for_write(TABLE_LIST *tables_used)
 {
   DBUG_ENTER("Query_cache::invalidate_locked_for_write");
-  if (query_cache_size > 0 && tables_used)
+  if (tables_used)
   {
     STRUCT_LOCK(&structure_guard_mutex);
-    if (query_cache_size > 0)
+    if (query_cache_size > 0 && !flush_in_progress)
     {
       DUMP(this);
       for (; tables_used; tables_used= tables_used->next_local)
@@ -1336,12 +1396,10 @@ void Query_cache::invalidate(THD *thd, TABLE *table,
 {
   DBUG_ENTER("Query_cache::invalidate (table)");
   
-  if (query_cache_size > 0)
-  {
   STRUCT_LOCK(&structure_guard_mutex);
-    if (query_cache_size > 0)
+  if (query_cache_size > 0 && !flush_in_progress)
   {
-      using_transactions = using_transactions &&
+    using_transactions= using_transactions &&
       (thd->options & (OPTION_NOT_AUTOCOMMIT | OPTION_BEGIN));
     if (using_transactions && 
         (table->file->table_cache_type() == HA_CACHE_TBL_TRANSACT))
@@ -1350,7 +1408,7 @@ void Query_cache::invalidate(THD *thd, TABLE *table,
       invalidate_table(table);
   }
   STRUCT_UNLOCK(&structure_guard_mutex);
-  }
+
   DBUG_VOID_RETURN;
 }
 
@@ -1359,20 +1417,18 @@ void Query_cache::invalidate(THD *thd, const char *key, uint32  key_length,
 {
   DBUG_ENTER("Query_cache::invalidate (key)");
   
-  if (query_cache_size > 0)
+  STRUCT_LOCK(&structure_guard_mutex);
+  if (query_cache_size > 0 && !flush_in_progress)
   {
-    using_transactions = using_transactions &&
+    using_transactions= using_transactions &&
       (thd->options & (OPTION_NOT_AUTOCOMMIT | OPTION_BEGIN));
     if (using_transactions) // used for innodb => has_transactions() is TRUE
       thd->add_changed_table(key, key_length);
     else
-    {
-      STRUCT_LOCK(&structure_guard_mutex);
-      if (query_cache_size > 0)
       invalidate_table((byte*)key, key_length);
-      STRUCT_UNLOCK(&structure_guard_mutex);  
-    }
   }
+  STRUCT_UNLOCK(&structure_guard_mutex);  
+
   DBUG_VOID_RETURN;
 }
 
@@ -1383,10 +1439,8 @@ void Query_cache::invalidate(THD *thd, const char *key, uint32  key_length,
 void Query_cache::invalidate(char *db)
 {
   DBUG_ENTER("Query_cache::invalidate (db)");
-  if (query_cache_size > 0)
-  {
   STRUCT_LOCK(&structure_guard_mutex);
-    if (query_cache_size > 0)
+  if (query_cache_size > 0 && !flush_in_progress)
   {
     DUMP(this);
   restart_search:
@@ -1414,7 +1468,7 @@ void Query_cache::invalidate(char *db)
     }
   }
   STRUCT_UNLOCK(&structure_guard_mutex);
-  }
+
   DBUG_VOID_RETURN;
 }
 
@@ -1422,15 +1476,14 @@ void Query_cache::invalidate(char *db)
 void Query_cache::invalidate_by_MyISAM_filename(const char *filename)
 {
   DBUG_ENTER("Query_cache::invalidate_by_MyISAM_filename");
-  if (query_cache_size > 0)
+
+  STRUCT_LOCK(&structure_guard_mutex);
+  if (query_cache_size > 0 && !flush_in_progress)
   {
     /* Calculate the key outside the lock to make the lock shorter */
     char key[MAX_DBKEY_LENGTH];
     uint32 db_length;
     uint key_length= filename_2_table_key(key, filename, &db_length);
-    STRUCT_LOCK(&structure_guard_mutex);
-    if (query_cache_size > 0)			// Safety if cache removed
-    {
     Query_cache_block *table_block;
     if ((table_block = (Query_cache_block*) hash_search(&tables,
       						  (byte*) key,
@@ -1438,7 +1491,7 @@ void Query_cache::invalidate_by_MyISAM_filename(const char *filename)
       invalidate_table(table_block);
   }
   STRUCT_UNLOCK(&structure_guard_mutex);
-  }
+
   DBUG_VOID_RETURN;
 }
 
@@ -1483,7 +1536,12 @@ void Query_cache::destroy()
   }
   else
   {
+    /* Underlying code expects the lock. */
+    STRUCT_LOCK(&structure_guard_mutex);
     free_cache();
+    STRUCT_UNLOCK(&structure_guard_mutex);
+
+    pthread_cond_destroy(&COND_flush_finished);
     pthread_mutex_destroy(&structure_guard_mutex);
     initialized = 0;
   }
@@ -1499,6 +1557,8 @@ void Query_cache::init()
 {
   DBUG_ENTER("Query_cache::init");
   pthread_mutex_init(&structure_guard_mutex,MY_MUTEX_INIT_FAST);
+  pthread_cond_init(&COND_flush_finished, NULL);
+  flush_in_progress= FALSE;
   initialized = 1;
   DBUG_VOID_RETURN;
 }
@@ -1694,6 +1754,17 @@ void Query_cache::make_disabled()
 }
 
 
+/*
+  free_cache() - free all resources allocated by the cache.
+
+  SYNOPSIS
+    free_cache()
+
+  DESCRIPTION
+    This function frees all resources allocated by the cache.  You
+    have to call init_cache() before using the cache again.
+*/
+
 void Query_cache::free_cache()
 {
   DBUG_ENTER("Query_cache::free_cache");
@@ -1728,17 +1799,51 @@ void Query_cache::free_cache()
   Free block data
 *****************************************************************************/
 
+
 /*
-  The following assumes we have a lock on the cache
+  flush_cache() - flush the cache.
+
+  SYNOPSIS
+    flush_cache()
+
+  DESCRIPTION
+    This function will flush cache contents.  It assumes we have
+    'structure_guard_mutex' locked.  The function sets the
+    flush_in_progress flag and releases the lock, so other threads may
+    proceed skipping the cache as if it is disabled.  Concurrent
+    flushes are performed in turn.
 */
 
 void Query_cache::flush_cache()
 {
+  /*
+    If there is flush in progress, wait for it to finish, and then do
+    our flush.  This is necessary because something could be added to
+    the cache before we acquire the lock again, and some code (like
+    Query_cache::free_cache()) depends on the fact that after the
+    flush the cache is empty.
+  */
+  while (flush_in_progress)
+    pthread_cond_wait(&COND_flush_finished, &structure_guard_mutex);
+
+  /*
+    Setting 'flush_in_progress' will prevent other threads from using
+    the cache while we are in the middle of the flush, and we release
+    the lock so that other threads won't block.
+  */
+  flush_in_progress= TRUE;
+  STRUCT_UNLOCK(&structure_guard_mutex);
+
+  my_hash_reset(&queries);
   while (queries_blocks != 0)
   {
     BLOCK_LOCK_WR(queries_blocks);
-    free_query(queries_blocks);
+    free_query_internal(queries_blocks);
   }
+
+  STRUCT_LOCK(&structure_guard_mutex);
+  flush_in_progress= FALSE;
+  pthread_cond_signal(&COND_flush_finished);
 }
 
 /*
@@ -1784,36 +1889,48 @@ my_bool Query_cache::free_old_query()
   DBUG_RETURN(1);				// Nothing to remove
 }
 
+
 /*
-  Free query from query cache.
-  query_block must be locked for writing.
-  This function will remove (and destroy) the lock for the query.
+  free_query_internal() - free query from query cache.
+
+  SYNOPSIS
+    free_query_internal()
+      query_block           Query_cache_block representing the query
+
+  DESCRIPTION
+    This function will remove the query from a cache, and place its
+    memory blocks to the list of free blocks.  'query_block' must be
+    locked for writing, this function will release (and destroy) this
+    lock.
+
+  NOTE
+    'query_block' should be removed from 'queries' hash _before_
+    calling this method, as the lock will be destroyed here.
 */
 
-void Query_cache::free_query(Query_cache_block *query_block)
+void Query_cache::free_query_internal(Query_cache_block *query_block)
 {
-  DBUG_ENTER("Query_cache::free_query");
+  DBUG_ENTER("Query_cache::free_query_internal");
   DBUG_PRINT("qcache", ("free query 0x%lx %lu bytes result",
 		      (ulong) query_block,
 		      query_block->query()->length() ));
 
   queries_in_cache--;
-  hash_delete(&queries,(byte *) query_block);
 
-  Query_cache_query *query = query_block->query();
+  Query_cache_query *query= query_block->query();
 
   if (query->writer() != 0)
   {
     /* Tell MySQL that this query should not be cached anymore */
-    query->writer()->query_cache_query = 0;
+    query->writer()->query_cache_query= 0;
     query->writer(0);
   }
   double_linked_list_exclude(query_block, &queries_blocks);
-  Query_cache_block_table *table=query_block->table(0);
+  Query_cache_block_table *table= query_block->table(0);
 
-  for (TABLE_COUNTER_TYPE i=0; i < query_block->n_tables; i++)
+  for (TABLE_COUNTER_TYPE i= 0; i < query_block->n_tables; i++)
     unlink_table(table++);
-  Query_cache_block *result_block = query->result();
+  Query_cache_block *result_block= query->result();
 
   /*
     The following is true when query destruction was called and no results
@@ -1827,11 +1944,11 @@ void Query_cache::free_query(Query_cache_block *query_block)
       refused++;
       inserts--;
     }
-    Query_cache_block *block = result_block;
+    Query_cache_block *block= result_block;
     do
     {
-      Query_cache_block *current = block;
-      block = block->next;
+      Query_cache_block *current= block;
+      block= block->next;
       free_memory_block(current);
     } while (block != result_block);
   }
@@ -1848,6 +1965,32 @@ void Query_cache::free_query(Query_cache_block *query_block)
   DBUG_VOID_RETURN;
 }
 
+
+/*
+  free_query() - free query from query cache.
+
+  SYNOPSIS
+    free_query()
+      query_block           Query_cache_block representing the query
+
+  DESCRIPTION
+    This function will remove 'query_block' from 'queries' hash, and
+    then call free_query_internal(), which see.
+*/
+
+void Query_cache::free_query(Query_cache_block *query_block)
+{
+  DBUG_ENTER("Query_cache::free_query");
+  DBUG_PRINT("qcache", ("free query 0x%lx %lu bytes result",
+		      (ulong) query_block,
+		      query_block->query()->length() ));
+
+  hash_delete(&queries,(byte *) query_block);
+  free_query_internal(query_block);
+
+  DBUG_VOID_RETURN;
+}
+
 /*****************************************************************************
  Query data creation
 *****************************************************************************/
@@ -2431,12 +2574,8 @@ Query_cache::allocate_block(ulong len, my_bool not_less, ulong min,
   if (!under_guard)
   {
     STRUCT_LOCK(&structure_guard_mutex);
-    /*
-      It is very unlikely that following condition is TRUE (it is possible
-      only if other thread is resizing cache), so we check it only after
-      guard mutex lock
-    */
-    if (unlikely(query_cache.query_cache_size == 0))
+
+    if (unlikely(query_cache.query_cache_size == 0 || flush_in_progress))
     {
       STRUCT_UNLOCK(&structure_guard_mutex);
       DBUG_RETURN(0);
@@ -2892,11 +3031,9 @@ static TABLE_COUNTER_TYPE process_and_count_tables(TABLE_LIST *tables_used,
   (query without tables are not cached)
 */
 
-TABLE_COUNTER_TYPE Query_cache::is_cacheable(THD *thd, uint32 query_len,
-					     char *query,
-					     LEX *lex,
-					     TABLE_LIST *tables_used,
-					     uint8 *tables_type)
+TABLE_COUNTER_TYPE
+Query_cache::is_cacheable(THD *thd, uint32 query_len, char *query, LEX *lex,
+                          TABLE_LIST *tables_used, uint8 *tables_type)
 {
   TABLE_COUNTER_TYPE table_count;
   DBUG_ENTER("Query_cache::is_cacheable");
@@ -2979,13 +3116,10 @@ my_bool Query_cache::ask_handler_allowance(THD *thd,
 void Query_cache::pack_cache()
 {
   DBUG_ENTER("Query_cache::pack_cache");
+
   STRUCT_LOCK(&structure_guard_mutex);
-  /*
-    It is very unlikely that following condition is TRUE (it is possible
-    only if other thread is resizing cache), so we check it only after
-    guard mutex lock
-  */
-  if (unlikely(query_cache_size == 0))
+
+  if (unlikely(query_cache_size == 0 || flush_in_progress))
   {
     STRUCT_UNLOCK(&structure_guard_mutex);
     DBUG_VOID_RETURN;
@@ -3300,7 +3434,7 @@ my_bool Query_cache::join_results(ulong join_limit)
   DBUG_ENTER("Query_cache::join_results");
 
   STRUCT_LOCK(&structure_guard_mutex);
-  if (queries_blocks != 0)
+  if (queries_blocks != 0 && !flush_in_progress)
   {
     DBUG_ASSERT(query_cache_size > 0);
     Query_cache_block *block = queries_blocks;
@@ -3587,31 +3721,23 @@ void Query_cache::tables_dump()
 }
 
 
-my_bool Query_cache::check_integrity(bool not_locked)
+my_bool Query_cache::check_integrity(bool locked)
 {
   my_bool result = 0;
   uint i;
   DBUG_ENTER("check_integrity");
 
-  if (query_cache_size == 0)
-  {
-    DBUG_PRINT("qcache", ("Query Cache not initialized"));
-    DBUG_RETURN(0);
-  }
-  if (!not_locked)
-  {
+  if (!locked)
     STRUCT_LOCK(&structure_guard_mutex);
-    /*
-      It is very unlikely that following condition is TRUE (it is possible
-      only if other thread is resizing cache), so we check it only after
-      guard mutex lock
-    */
-    if (unlikely(query_cache_size == 0))
+
+  if (unlikely(query_cache_size == 0 || flush_in_progress))
   {
+    if (!locked)
       STRUCT_UNLOCK(&query_cache.structure_guard_mutex);
+
+    DBUG_PRINT("qcache", ("Query Cache not initialized"));
     DBUG_RETURN(0);
   }
-  }
 
   if (hash_check(&queries))
   {
@@ -3856,7 +3982,7 @@ my_bool Query_cache::check_integrity(bool not_locked)
     }
   }
   DBUG_ASSERT(result == 0);
-  if (!not_locked)
+  if (!locked)
     STRUCT_UNLOCK(&structure_guard_mutex);
   DBUG_RETURN(result);
 }

sql/sql_cache.h

@@ -195,7 +195,6 @@ extern "C"
   byte *query_cache_table_get_key(const byte *record, uint *length,
 				  my_bool not_used);
 }
-void query_cache_insert(NET *thd, const char *packet, ulong length);
 extern "C" void query_cache_invalidate_by_MyISAM_filename(const char* filename);
 
 
@@ -241,6 +240,12 @@ class Query_cache
   ulong free_memory, queries_in_cache, hits, inserts, refused,
     free_memory_blocks, total_blocks, lowmem_prunes;
 
+private:
+  pthread_cond_t COND_flush_finished;
+  bool flush_in_progress;
+
+  void free_query_internal(Query_cache_block *point);
+
 protected:
   /*
     The following mutex is locked when searching or changing global
@@ -249,6 +254,12 @@ class Query_cache
     LOCK SEQUENCE (to prevent deadlocks):
       1. structure_guard_mutex
       2. query block (for operation inside query (query block/results))
+
+    Thread doing cache flush releases the mutex once it sets
+    flush_in_progress flag, so other threads may bypass the cache as
+    if it is disabled, not waiting for reset to finish.  The exception
+    is other threads that were going to do cache flush---they'll wait
+    till the end of a flush operation.
   */
   pthread_mutex_t structure_guard_mutex;
   byte *cache;					// cache memory
@@ -358,6 +369,7 @@ class Query_cache
     If query is cacheable return number tables in query
     (query without tables not cached)
   */
+  static
   TABLE_COUNTER_TYPE is_cacheable(THD *thd, uint32 query_len, char *query,
 				  LEX *lex, TABLE_LIST *tables_used,
 				  uint8 *tables_type);
@@ -410,6 +422,7 @@ class Query_cache
 
   void destroy();
 
+  friend void query_cache_init_query(NET *net);
   friend void query_cache_insert(NET *net, const char *packet, ulong length);
   friend void query_cache_end_of_result(THD *thd);
   friend void query_cache_abort(NET *net);
@@ -435,6 +448,8 @@ class Query_cache
 
 extern Query_cache query_cache;
 extern TYPELIB query_cache_type_typelib;
+void query_cache_init_query(NET *net);
+void query_cache_insert(NET *net, const char *packet, ulong length);
 void query_cache_end_of_result(THD *thd);
 void query_cache_abort(NET *net);
 

sql/sql_class.cc

@@ -223,7 +223,7 @@ THD::THD()
 #endif
   client_capabilities= 0;                       // minimalistic client
   net.last_error[0]=0;                          // If error on boot
-  net.query_cache_query=0;                      // If error on boot
+  query_cache_init_query(&net);                 // If error on boot
   ull=0;
   system_thread= cleanup_done= abort_on_warning= no_warnings_for_error= 0;
   peer_port= 0;					// For SHOW PROCESSLIST